To print effectively and efficiently using a color thermal printing process, the dye impregnated donor web must be properly positioned relative to the dye receiver. Proper positioning is required to ensure full coverage of the image area by successive color patches. A typical color donor web contains a repeating series of yellow, magenta and cyan color patches, and in some cases, a black patch and/or a clear fusing patch. Each patch must be properly aligned with the receiver to ensure high quality printing. One way to align or index the receiver and donor is by using a detector which will detect whether the color is yellow, magenta, cyan, black or clear, and identify its position.
In some donor sensing arrangements, a donor web is encoded along the edges with marks that are detected by a detecting means. The general alignment is obtained with the sensor elements placed in the donor path past the thermal print head so that the marks are detected after the donor emerges from the print head. Accordingly, it will be appreciated that it would be highly desirable to detect the markings before the donor web exits the printing area.
In other sensing arrangements, the donor web is located in a cassette with an opening therein for engagement with a sensor. The donor passes by the sensor a point that is a relatively long distance from the area where printing occurs. The color sensor senses the color of the donor as the donor is unwound from the donor supply spool before printing occurs. Understandably, much could happen between the location of the sensor and the printing location while the donor ribbon traverses this course. Accordingly, it will be appreciated that it would be highly desirable to have a color sensor to accurately sense the position and color of the donor ribbon close to the printing area for accurate registration of the colors during printing.
U.S. Pat. No. 4,710,781 which issued Dec. 1, 1987 to Stanley W. Stephenson and reissued on Jul. 10, 1990, as U.S. Pat. No. 33,260, discloses an apparatus for identifying different color frames of a donor web. A sensor includes a light emitting diode (LED) to emit red or yellow light and a corresponding photodetector to respond to the red or yellow light. A space saving arrangement positions two LEDs to illuminate the same spot on the donor web adjacent an edge of the web. The yellow and red light pass through dye frames of the moving donor web and illuminate the appropriate photodetectors. The general alignment is obtained with the sensor elements placed in the donor path past the thermal print head so that color frames are detected after the donor emerges from the print head. Where frames are detected after printing, there is an amount of each frame, equal to the length of donor between the print head and detectors, that is wasted. Accordingly, it will be appreciated that it would be highly desirable to have sensors to accurately sense the position and color of the donor web at the print line for accurate registration of the colors during printing and to thereby minimize wasted donor.